408 THE PROTOZOA 



The amcebulse next associate in couples. In each couple the cytoplasm of 

 the associates undergoes partial fusion. In one associate the loosely-textured 

 nucleus remains unaltered ; in the other the nucleus divides by mitosis, and 

 one of the two daughter-nuclei, with a small quantity of the cytoplasm, is 

 cast off. The bodies of the two associates now fuse completely into a single 

 mass containing two nuclei, a large and a small. The larger nucleus represents 

 the unaltered nucleus of one of the two original associates ; the smaller 

 nucleus is the reduced nucleus of the other associate. 



The binucleate stage formed by the association of two amcebulse (planonts) 

 is the foundation of the plasmodium. The two nuclei remain separate and 

 multiply independently, maintaining their difference in size. Hence the 

 young plasmodia consist of nuclei of two sizes, small and large ; and this 

 difference in size, which has often been noted in the plasmodia of other 

 species, is due, according to Auerbach, to the fact that the smaller nuclei are 

 derived from a nucleus which has undergone reduction, while the larger nuclei 

 are derived from one which has not done so. 



So far as it is possible to generalize from the few recorded observa- 

 tions and experiments, the germination of the Myxosporidian spore 

 probably takes place always in the digestive tract of the new host. 

 The first act in the process is the rounding off of the amoeboid 

 sporozoite ; next the polar capsules are discharged, the function of 

 these organs being, apparently, that of fixing the spore to the wall 

 of the gut. Then the two valves of the sporocyst separate, and 

 the amoebula creeps out ; its two nuclei copulate to form the syn- 

 karyon, if they have not done so already. In this way the planont- 

 phase arises from the sporozoite, and is set free in the digestive 

 tract, whence it migrates to the organ or tissue of which it is a 

 specific parasite. When its destination is an organ which, like the 

 gall-bladder, is in open communication with the gut, the migration 

 may be comparatively simple and direct ; but in the majority of 

 cases the journey to be accomplished is a complicated one. It is 

 probably safe to assume that in most cases the planont passes 

 through the wall of the gut into the channels of the blood or lymph, 

 and by this route arrives ultimately at its destination. In the 

 organ or tissue which the parasite attacks, the planont probably 

 passes through a period of cell-parasitism and proliferation by 

 binary fission ; ultimately the plasmodial phase is initiated by 

 association of two planonts, of which the bodies fuse, but the nuclei 

 remain separate. Growth of the body with independent multipli- 

 cation of its nuclei to four (Disporea) or many (Polysporea) produces 

 the spore-forming plasmodium characteristic of the order. 



The Myxosporidia are classified as follows : 



SUBORDER I. : DISPOREA. Only two spores formed in the plasmodium. 

 The greatest diameter of the spore is at right angles to the sutural plane 

 (Fig. 169). 



One family, Ceratomyxidce, with two genera : Ceratomyxa, with several 

 species parasitic in the gall-bladders of fishes; Leptotheca (Fig. 165), in- 

 cluding several species from gall-bladders of fishes, but L. ranarum occurs 

 in the kidneys of frogs (Eana spp.). 



